The present invention relates to a semiconductor device, and more particularly to an electrode structure of a semiconductor device having a control electrode arranged at the center of one major surface of a semiconductor substrate.
Electrodes are brazed to a semiconductor substrate to allow the flow-in and flow-out of current to and from the semiconductor substrate. If a thermal expansion cofficient of the semiconductor substrate does not coincide with that of the electrode, a thermal stress is applied to the brazing material, resulting in thermal fatigue which in turn damages the solderbility and deteriorates the characteristic of the semiconductor device. Accordingly, in the semiconductor device, it is necessary that the thermal expansion coefficient of the semiconductor substrate coincides with that of the electrode.
The semiconductor substrate of the semiconductor device is usually hermetically sealed to prevent the characteristic of the semiconductor device from being varied with the change of an external atmosphere or to prevent the semiconductor device from being damaged by an external impact.
The hermetic seal structures are basically classified into two major categories. In one category the semiconductor substrate is accommodated in a casing having a portion thereof made of solid insulator such as ceramics, and in the other category the semiconductor substrate is molded by synthetic resin such as epoxy resin. The latter is more advantageous than the former from an economic standpoint because the cost needed for sealing is much smaller.
However, because of incompleteness of sealing, the synthetic resin sealed semiconductor device is less reliable in its characteristic. As a result, it is needed to apply a more reliable passivation to the semiconductor substrate. An example of high passivation material is glass. However, because of unequality between the thermal expansion coefficient of the semiconductor substrate and that of the electrode, the passivation glass will be broken when a thermal stress is generated because the glass has a small bending strength. As a result, the passivation function will be lost.
Since the degree of bend depends on the soldering area of the semiconductor substrate and the electrode, the semiconductor substrate must be of small area in order to prevent the passivation glass from being broken.
A current carrying capability of the semiconductor device is proportional to the area of the semiconductor substrate. Accordingly, the semiconductor device having the glass passivation and molded by synthetic resin has not been put into practice in a power semiconductor device which requires a high current carrying capability.
Some of the semiconductor devices, such as transistors and thyristors each has a pair of main electrodes for conducting a main current as well as a control electrode which, in many cases, is arranged on one major surface of the semiconductor substrate on which one of the main electrodes is arranged. The control electrode is arranged in one of two major ways: In one way the control electrode is arranged on a periphery of one major surface, and in the other way the control electrode is arranged at the center of the major surface. In the latter case, a complex structure is needed because the control electrode must be electrically insulated from the main electrode and yet they must be mechanically joined together.